This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. MAP.3B is a companion study to the NCIC CTG MAP.3 core study &&#35;40;project&&#35;35;11794-01&&#35;41;. The core study is evaluating whether an aromatase inhibitor, exemestane, will reduce breast cancer incidence in postmenopausal women. All women who enroll in the MAP.3 core study will be screened for MAP.3B companion study eligibility at their MAP.3 core study initial screening visit and, if eligible, invited to participate.. This trial will be conducted in Canada and the United States and about 480 women will take part in this study. The study objectives are: 1. to assess the percentage change of Bone Mineral Density &&#35;40;BMD&&#35;41;of the spine and total hip at one and five years from baseline after randomization to the core protocol;2. to compare the proportion of women who develop BMD of the spine or total hip below the absolute threshold value for osteoporosis in the treatment groups and 3. to compare the long term clinical safety of exemestane with respect to osteoporosis. The effect on fracture incidence between exemestane and placebo will be evaluated on the entire MAP.3 population as part of the core protocol. MAP.3B subject population: MAP.3 subjects who have a Bone Mineral measurement &&#35;40;using DEXA&&#35;41;with a BMD T-score >&&#35;61;-1.9 SD, done within 8 weeks prior to randomization to the MAP.3 core protocol, may participate in the MAP.3B companion protocol. MAP.3B additional procedures: Taking part in MAP.3B study, two additional BMD measurements &&#35;40;using DEXA&&#35;41;of hip and spin will be performed at two and five years following enrollment. Two additional blood samples will be collected for bone biomarker tests at one and five years following enrollment. This is a low risk study. There are no known described risks or side effects to the BMD measurements with DEXA scan. The subject will receive a very low dose of radiation from DEXA scan and the chance of this scan causing cancer is very small. The effects of drawing blood may cause pain, bleeding or bruise where the needle is inserted. To conduct MAP.3B study, a MAP.3B special informed consent form and PHI will be used for consenting and all MAP.3B related study documents will be kept in a separate study regulatory binder. Based on NCIC CTG instruction, MAP.3B companion study does not need a separate FDA F1572, Finacial disclosure form and investigator study acknowledgement form. We may use the same screening and enrollment log as MAP.3 core study use.